Technical Ignition coil not receiving correct voltage


New member
Apr 5, 2020
Hi all, I'm two separate issues involving the ignition coil wiring and the alternator on my 126. I'll try my best to outline the problems I'm seeing.

1. Ignition issue:
My car's been having an issue which I've narrowed down to ignition coil overheating. The symptoms were basically trouble hot starting, severe misfiring after driving around for a bit, and a coil too hot to touch after driving around for a bit. The original coil looks really old so I swapped it out for one which was used but I believe working and it had the same issues.

While going around checking the ignition side of the engine (points gap, spark plug gap, all fine), I noticed that when I hooked up my timing light to the coil terminals it was malfunctioning. A further test with my multimeter indicated that there was only about 4V across the coil, dropped down to 3V with higher RPMs, so my theory is for some reason the coil was getting much lower voltage than it should which was causing it to overheat by drawing too much current.

I tried testing where the remaining 8V was going and basically found that when I had my multimeter between the negative terminal of the coil and earth it was showing that 8V. I then went around to seeing what the resistance was and basically found the following:

Between terminals of coil: 3 Ohms (normal I believe)
Between positive terminal of coil and earth: 16 Ohms
Between negative terminal of coil and earth: 20 Ohms

But when testing voltage between the positive terminal of the coil and earth it was showing the full ~13V.

From what I understand, it looks like the full 12V is going in from the battery into the positive terminal of the coil, the coil is drawing 4V, then somehow from the negative terminal to the distributor it's drawing 8V. I have also gone and installed an electronic ignition system (Accuspark) in the distributor in case if it was the points playing up, it's now drawing 2V across the coil when engine is running.

Can anyone shed some light onto this and where I might be having an issue? From what I understand it should be 12V or so across the coil terminals right?

For reference in case if I've done something stupid with the wiring, the positive side of the coil is connected to a orange wire which is the 12V from the battery and a capacitor/condenser mounted on the stud holding the coil on the body. The negative side of the coil is only connected to the points on the distributor.

2. Alternator issue:
This is a separate issue which I've stupidly made myself. Basically while I was diagnosing the above, I wanted to check that my timing light was working and my timing was all fine. Since the coil was not getting 12V, I decided to try connect the light up to the alternator (I have a Marelli-type alternator installed).

Unfortunately the clip from the light slipped once I put it on the alternator stud for positive and it shorted with the alternator pulley, resulting in a spark and some sizzling sounds from near the alternator. I took it off once I saw that, but by the time I got it off it wasn't trying to short anymore.

After that I tried to measure the voltage from the positive of the alternator to earth and it looks like it's basically only got 3V or so, as opposed to the healthy ~12-13V it had previously. Also testing for continuity with the battery showed that there was no clear connection between the alternator and the negative terminal of the battery. I had a look at the fusebox and no fuses were blown, no obvious signs of wires blowing or anything either.

What would shorting this have done to the circuit? I had a look at the wiring diagram I have, although it's for a dynamo it shows that the circuit only goes through the voltage regulator before directly going to the battery, would this have fried the voltage regulator, some wiring to the battery, or maybe something internal on the alternator?

I will note also that the car still starts fine, so I know the starter is working fine, but I have a strong feeling it's not charging. The indicators don't work at all when there's other electrics drawing power (i.e. if brakes or lights are on it doesn't even try to blink).

Anyone able to shed some light on any of these two I've outlined above? Any help or thoughts would be much appreciated, thanks!